This invention relates to fuel humidification for use in high temperature fuel cell systems and which can also be used in conjunction with water recovery.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions. High temperature fuel cells, such as molten carbonate fuel cells, operate by passing a reactant fuel gas through the anode, while oxidant gas containing carbon dioxide and oxygen is passed through the cathode.
Reactant gases supplied to the fuel cell, and in particular, the reactant fuel gas supplied to the anode, must be sufficiently humidified to maintain a desired carbon to steam ratio in the fuel for proper and efficient fuel cell operation, to provide sufficient ionic conductivity of the reactant gases and to prevent carbon deposition, or soot formation, in the fuel cell. The amount of water consumed by fuel cell system for humidifying reactant gases is usually significant and requires a continuous supply of water to the fuel cell system. Accordingly, water management in fuel cell systems is very important and has a significant impact on fuel cell performance and operating efficiencies.
Water for humidifying reactant gases may be supplied from an external water supply source or may be recycled from anode and/or cathode exhaust gases by separating water vapor produced by the electrochemical reaction in the fuel cell. Examples of water recycling from the anode exhaust gas are disclosed in U.S. Pat. Nos. 5,068,159 and 4,039,579, which teach using a cooler and condenser to separate water from the anode exhaust stream, and thereafter passing the separated water through a boiler and a heater and feeding the water to the inlet of the anode compartment.
Another U.S. Pat. No. 7,060,382, assigned to the same assignee herein, discloses a system in which a water transfer assembly in the form of a partial-pressure swing water transfer wheel is used to separate water vapor in anode exhaust as water to the fuel feed. This patent also discloses a system in which this transfer is carried out by using heat exchangers, where the anode exhaust is cooled by the oxidant supply gas, water recycle vaporization and/or a cooling water or an ambient-air cooled heat exchanger (air fan). After being cooled, the stream is fed to a scrubbing and blow-down assembly where electrolyte contaminated water is removed. The resultant stream is then further cooled in a heat exchanger using an air fan or cooling water and the stream is then fed to a condensing unit. In this unit, the water is removed and fed to the fuel feed, while the stream is further fed to a carbon dioxide transfer assembly.
Fresh water used for humidification of the reactant gases in a fuel cell system is usually in the form of steam and must be free of electrolyte and other volatile and non-volatile contaminants. Therefore, fresh water supplied to the fuel cell system usually has to be treated in a water treatment system before it is suitable for humidifying the reactant gases, and in particular, the fuel supplied to the fuel cell. Such water treatment systems require complex, expensive high-pressure drop water treatment equipment to remove contaminants from the feed water. Moreover, conventional humidification systems employ heat exchanger equipment which is expensive and/or has relatively low reliability.
It is therefore an object of the present invention to provide a fuel humidification assembly which has a simpler and more reliable design.
It is also an object of the present invention to provide a fuel humidification assembly which does not require the conventional water treatment and high pressure drop equipment.
It is a further object of the present invention to provide a fuel humidification assembly which results in greater manufacturing and operating efficiencies in the fuel cell system.